Ozone Formation in Coastal Urban Atmospheres: The Role of Anthropogenic Sources of Chlorine

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1 Ozone Formation in Coastal Urban Atmospheres: The Role of Anthropogenic Sources of Chlorine, Sarah Oldfield, Charles B. Mullins, David T. Allen In this communication, we present experimental results from our environmental chamber studies to suggest that anthropogenic sources of molecular chlorine (Cl 2 ), a photolytic source of Cl, may contribute significantly to ozone formation in some urban environments. Many studies have focused on the sources and chemistry of halogen atoms in pristine marine environments 1-3, often attributing a net consumption of ground-level ozone to reactions with halogen atoms, primarily bromine and to a lesser extent, chlorine 4-7. However, little attention has been directed at determining the effect of Cl on ozone formation in urban environments, where oxides of nitrogen (NO x ) and volatile organic compounds (VOCs) are ubiquitous and there exist many anthropogenic sources of Cl 2. Large anthropogenic sources of Cl 2 emissions include chemical production facilities, water treatment plants, and paper production operations. Chlorine atoms, formed by the photolysis of Cl 2, react with alkanes at up to two orders of magnitude faster than do hydroxyl radicals and can promote the formation of O 3 in the presence of VOCs and NO 2,3 x. To understand how chlorine promotes ozone formation in a mixture of VOCs and NO x representative of the coastal urban troposphere, we performed experiments in outdoor Teflon environmental chambers. The chambers were approximately 2 m 3 in volume with internal volume to surface ratios of approximately 0.13 m when fully inflated. The chambers were conditioned 8,9 and subsequently prepared by flushing with zero air overnight. A commercially available mixture of 56 hydrocarbons (Matheson Enviro-Mat Ozone Precursor), as well as individual hydrocarbon reactants were used in the experiments. NO x (Praxair- NO/NO 2 at a ratio Page 1 of 6

2 of 200:1) was also injected into the chamber while the chamber was covered with an opaque tarp. After these reactants were allowed sufficient time to mix, Cl 2 (Air Products and Chemicals) was injected, the tarp removed, and gas sampling begun. Gas withdrawn from the chamber was delivered to O 3 (Dasibi 1008AH or 1003PC) and NO x (Monitor Laboratories 9841 or Columbia Scientific Industries 1600) analyzers. These continuous measurements were collected as 5- minute averages by Climatronics Corporation IMP 850 microloggers. Air samples for hydrocarbon analysis were collected in 6-liter stainless steel Summa canisters and analyzed by a HP 5890A gas chromatograph (GC) equipped with a flame ionization detector (FID) and/or a HP 6890 GC with a HP 5972 mass selective detector and Entech 7000 preconcentrator/cryofocuser. A first set of chamber experiments was directed at showing whether the addition of Cl 2 to a mixture of VOCs and NO x representative of conditions found in Houston, TX would promote the formation of O 3. Initial reactants included a mixture of 56 hydrocarbons with a total hydrocarbon concentration of approximately 1 part per million carbon (ppm c ). Sufficient NO x was injected to yield an initial VOC/NO x ratio of 10 ppb c /ppb v. Initial Cl 2 concentrations were between 0 and 47 ppb v. Each run was conducted under conditions of similar solar flux and temperature. The data summarized in Table 1 show that the peak O 3 concentration ([O 3 ] peak ) increases by up to a factor of six with the addition of Cl 2, and the time required to reach 0.63*[O 3 ] peak was reduced by up to a factor of 3.5. In addition, experiments that included injections of Cl 2 showed significant losses of alkanes, specifically substituted alkanes. The loss of alkanes increased from less than 10 percent in experiments without added Cl 2 to greater than 60 percent for hexane, 2,3-dimethylpentane, and octane in experiments with added Cl 2. Losses of alkenes and aromatic compounds were similar between runs with and without added Cl 2 (Table 2). If the chemistry in these experiments were dominated by the reactions of hydroxyl Page 2 of 6

3 radical, loss of alkenes and substituted aromatics would be expected, but the C 6+ alkanes would be relatively unreactive and contribute little to the formation of ozone 16. What was observed, however, was a depletion of alkanes and an increase in ozone concentrations. This is consistent with chlorine radical initiated oxidation chemistry. A second set of chamber experiments were run to identify which classes of hydrocarbons were most significantly affecting O 3 production associated with the addition of Cl 2. During these experiments n-pentane (EM Omnisolv), propene (Matheson-C.P. grade), and benzene (EMreagent grade) were used to represent each of the major classes (alkanes, alkenes, and aromatics) of hydrocarbons found in the urban troposphere. In runs with approximately equal concentrations (in ppb c ) of benzene, pentane, and propene, no significant changes in peak O 3 concentration or rate of formation were observed with the addition of Cl 2. However, for runs containing only pentane in air, a significant increase in peak O 3 concentration was observed and the activation time for significant ozone production decreased by a factor of 2. To a first approximation, these results can be explained by comparing the rates of reaction of OH and Cl with each of the reactant hydrocarbon species (Table 3). The reaction rate constant of OH with propene is approximately an order of magnitude greater than the reaction rate constants of OH with pentane or benzene. However, the reaction of Cl with pentane and propene proceed with similar rate constants at 298K. Benzene reacts even more slowly with Cl than OH at this temperature. Because anthropogenic O 3 formation in the presence of VOCs and NO x is initiated by reactions of free radicals with hydrocarbons and subsequent conversion of NO to NO 2, Cl appears to have a pronounced effect on O 3 production Page 3 of 6

4 and peak concentrations when there exists significant Cl activation of alkanes relative to alkene oxidation by OH and Cl. Our initial findings are presented as evidence to suggest that Cl 2 can significantly contribute to ozone formation in urban areas. By adding Cl 2 to a mixture of VOCs and NO x in air typical of urban atmospheres, we show that the rate of formation and peak concentrations of ozone significantly increase relative to control runs without added molecular chlorine. In addition to an increase in O 3 formation, Cl dominated chemistry is evidenced by the significant loss of alkanes, typically not observed for chemistry dominated by OH. Further environmental chamber studies will be performed to determine whether these phenomena occur at various VOC/NO x ratios and with different VOC compositions. Acknowledgements The authors wish to thank the Texas Natural Resource Conservation Commission for their generous support of our investigative work. The authors would also like to acknowledge Dr. Gary Vliet and Ian Bird of the Solar Energy Laboratory at the University of Texas at Austin for providing solar flux data. References 1. C. W. Spicer et al., Nature, 394, 353 (1998). 2. K. W. Oum, M. J. Lakin, D. O. DeHaan, T. Brauers, B. J. Finlayson-Pitts, Science, 279, 74 (1998). 3. B. J. Finlayson-Pitts, Res. Chem. Intermed. 19, 235 (1993). 4. B. J. Finlayson-Pitts and J. Pitts Jr., Science, 276, 1045 (1997). Page 4 of 6

5 5. B. Ramacher, J. Rudolph, R. Koppman, J. Geophys. Res., 104, 3633 (1999). 6. M. O. Andreae and P. J. Crutzen, Science, 276, 1052 (1997). 7. R. Vogt, P. J. Crutzen, R. Sander, Nature, 383, 327 (1996). 8. B. J. Finlayson-Pitts and J. N. Pitts Jr., Atmospheric Chemistry: Fundamentals and Experimental Techniques (Wiley, New York, 1986). 9. D. Grosjean, Env. Sci. Tech., 19, 1059 (1985). 10. R. Atkinson et al., J. Phys. Chem. Ref. Data, 28, 191 (1999). 11. R. Atkinson, J. Phys. Chem. Ref. Data, 26, 215 (1997). 12. R. Atkinson, Gas-phase Tropospheric Chemistry of Organic Compounds, (Monogr. 2, J. Phys. Chem. Ref. Data, American Chemical Society, Washington D.C., 1994) pp P. Ariya, thesis, York University (1996). 14. R. Atkinson and S. M. Aschmann, Int. J. Chem. Kin., 17, 33 (1985). 15. W. B. DeMore et al., JPL Publ. No (1997). 16. W. P. L. Carter, J. A. Pierce, I. L. Malkina, Atm. Env., 29, 2499 (1995). Page 5 of 6

6 Table 1 Ozone Data Summary Hydrocarbon (HC) Mixture HC/NO x (ppb c /ppb v ) [Cl 2 ] o (ppb v ) [O 3 ] Peak (ppb v ) Time to reach 0.63*[O 3 ] Peak (min.) (O 3 -NO) Cl 2 O 3 Precursor na O 3 Precursor O 3 Precursor O 3 Precursor Average: 4.9 PBP * na PBP * * Pentane na Pentane * PBP Pentane, benzene, and propene. ** No increase in peak ozone concentration observed.

7 Table 2 Fractional Losses of VOCs: Experiments with O 3 Precursor Compound Fraction Lost [Cl 2 ] o = 0 ppb v Fraction Lost [Cl 2 ] o = 14 ppb v Fraction Lost [Cl 2 ] o = 20 ppb v Fraction Lost [Cl 2 ] o = 47 ppb v Alkanes 2,3-Dimethylpentane n-hexane n-octane n-nonane Alkenes 1-Pentene Methyl, 1-3-Butadiene Aromatics Benzene Toluene

8 Table 3 Rate Constants Compound k OH (10-14 cm 3 molecule -1 s -1 ) k Cl (10-14 cm 3 molecule -1 s -1 ) Alkanes Methane (10) Ethane (10) 25 5,900 Propane (10) ,000 n-butane 244 (11) 22,000 (10) n-pentane (11) ,000 n-hexane (11) ,000 Alkenes Ethene 900 (10) 10,700 (11) Propene 3,000 (10) 28,000 (11) Aromatics Benzene 111 (12) 0.9 (13) Toluene 5,960 (12) 5,890 (14)

ATMOSPHERIC CHEMISTRY OF SELECTED HYDROXYCARBONYLS. Sara M. Aschmann, Janet Arey and Roger Atkinson

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